This invention relates to the preparation of glass containers for thermoplastic membrane sealing. Heat activated membrane seals for plastic containers are common in the packaging and canning art. Generally a membrane, which may be a laminate of aluminum foil and a thermoplastic polymer, is pressed on the rim of a plastic container and heated to form polymer-polymer adhesive contact. The problem with this method when used with soda-lime-silica glass containers is that the failure of the seal will generally occur within a few days of sealing due to poor initial adhesion between the glass and the polymer or poor durability of the adhesion.
Substantially improved adhesion between a membrane seal and a container finish is obtained by subjecting the finish to a high temperature treatment which modifies the finish surface. This high temperature treatment preferably is conducted immediately after the containers are formed and comprises contacting the finish with a decomposible fluorine-containing compound; a compound which decomposes to form a metal oxide; a compound which decomposes to form a sulfur oxide; sulfur oxides; and combinations of the above. These treating materials may be used in any order or combination, or the metal oxide precursor may be used alone.
A sealing closure for the container is formed by pressing a membrane comprising a thermoplastic film onto the container finish and heating to form a glass/plastic adhesive bond. The membrane desirably may be an aluminum foil-thermoplastic film laminate. It may also be a polymer sheet, a laminate of polymer, a paper/polymer laminate or a laminate of one or more layers of polymers, metal foil, and paper.
This described technique provides a reliable and secure closure so long as the container finish remains uncontaminated. However, during cold end treatment of the containers after annealing, it was found that the treating material, typically an organic polymer, stearate or silicone, would occasionally be deposited on the container finish to the extent that erratic sealing results were obtained.